Endothelin is a cyclic peptide which possesses a strong, long-lasting vasoconstricting effect, and is thought to be one of the substances responsible for hypertension, asthma, acute renal failure, cardiac infarction, cerebral apoplexy, angina pectoris and cerebral vasospasm. Consequently, a substance which antagonizes endothelin and inhibits its effects is expected to be useful for the treatment and prevention of these diseases.
It is known that the cyclic peptide represented by the formula (A) SEQ ID NO:1 ##STR2## (wherein U represents D-Val or D-allo-Ile, R.sup.1 represents hydrogen or an amino-protective group, and R.sup.2 represents hydrogen or a carboxyl-protective group) exhibits the endothelin antagonism (Japanese Published Unexamined Patent Application No. 130299/91).